1. Field of the Invention
The present invention relates to a light emitting apparatus, an image forming apparatus, a display apparatus, and an image pickup apparatus incorporating organic EL (electroluminescent) elements.
2. Description of the Related Art
There have recently been increasing demand for organic EL displays with reduced power consumption and growing expectation for organic EL elements with improved emission efficiency. An organic EL element is composed of a light-reflective electrode, an organic compound layer, and an optically transmissive electrode stacked on top of one another, and its emission efficiency can be improved by adjusting the thickness of the organic compound layer and other characteristics and utilizing the resultant light interference effect, for example (Japanese Patent Laid-Open No. 2005-285395).
More specifically, the optical length L between the light-reflective electrode and the emission point of the organic EL element is set to satisfy formula 1, where λ is the wavelength to be enhanced, Φ is the sum of the phase shifts that occur when the light reflects off the light-reflective electrode, and m is an integer of zero or greater. The light interference effect is maximized when m is zero.L={2m−(Φ/π)}×(λ/4)  Formula 1
A typical configuration of the light-reflective electrode is that found in Japanese Patent Laid-Open No. 2005-285395, i.e., a laminate consisting of a 100-nm Al-based metallic reflective sheet and a 100-nm ITO cathode formed on the metallic sheet. This publication also discloses that the contact resistance between the Al-containing metallic compound and ITO is high and thus the cathode may be directly connected to a thin film transistor (TFT) via a contact hole so that the organic EL element can operate.
However, making the thickness of such an ITO layer as large as 100 nm causes the organic compound layer to be too thin to completely cover the protrusions and depressions or any foreign particulate matter on the surface of the light-reflective electrode when the optical length L of the organic EL element is set to satisfy formula 1 with m equal to zero. A short-circuit or current leakage would occur between the electrodes, and the organic EL element would become incapable of emitting light.
A possible solution to this problem is to reduce the thickness of the ITO layer, but this also leads to an increased sheet resistance of the ITO layer and an accordingly increased driving voltage of the organic EL element.
Omitting the ITO layer causes the surface of the Al-based metallic layer to be more susceptible to oxidation. The Al-based metallic layer would be coated with aluminum oxide during the etching process for forming an electrode pattern or while the substrate and electrode surfaces are cleaned. The insulating aluminum oxide layer would interfere with the injection of charges into the organic compound layer, increasing the driving voltage of the organic EL element.